1. Field of the Invention
This invention relates to image processing and map production systems. The invention is especially directed to the processing of images or data derived by remote-sensing systems, which may be, for example, satellite-borne or airborne, and in which the imaging or data is suitably recorded in digital form. In the context of the present disclosure therefore, the terms "satellite image" and "satellite data" are to be understood as encompassing also images or data furnished by other remote-sensing systems and especially by airborne remote-sensing systems, while the term "remote-sensing" is to be interpreted as embracing satellite-borne and airborne imaging and data collection techniques for recording information relating to geographical areas or regions. In particular, the invention relates to the processing of satellite images and the preparation of maps from such images. The invention also extends to integration of data from conventional maps with satellite image data, to provide a satellite image map.
2. Description of the Prior Art
Traditional methods of mapping are known over a long number of years and are highly developed. They allow extreme accuracy over small areas, but are relatively costly to apply over larger regions. Thus by and large only developed areas of the world have been mapped with any high degree of accuracy, as detailed surface measurements are not readily practicable in unpopulated, difficult and inaccessible country. In more recent times, aerial photography has greatly enlarged the scope for accurate mapping over larger areas, while satellite imaging (space-borne remote sensing) now offers even better opportunities for surveying virtually any part of the world, irrespective of its accessibility or otherwise.
Space-borne remote sensing systems may be either passive or active. In a passive system, naturally available radiation in the form of reflected light or emitted ambient thermal energy is recorded. Reflection represents radiation that is neither absorbed nor transmitted. Since the extent to which energy is reflected, absorbed or emitted is different for different objects and also depends on wavelengths, by measuring reflection, absorption and emission, appropriate parameters may be determined to distinguish between different environmental features. Objects that appear similar in one waveband may be quite distinguishable in another. In the visible spectrum, these variations are perceived as colour.
In an active system, an artificial energy source is used, for example light, radar pulses or laser beams, and the returned signal or echo of a transmitted energy beam aimed at an object gives a measure of reflectance or absorption.
Remote sensing techniques allow either single broadband imaging, as in panchromatic photography, or objects are observed simultaneously in a number of wavelength bands in a multi-spectrum technique. A combination of passive multi-spectral scanning and active microwave imaging is used in some of the more successful satellite systems. While certain passive systems have a relatively coarse spatial resolution, radar-type developments of active systems can yield spatial resolutions down to approximately 10 meters, resolution being defined here as the minimum distance between two objects at which they still appear distinct and separate on an image or photographic representation. Objects which are closer together than the resolution limit appear as a single object. In digital work, resolution is also defined in terms of the pixel size of scan. This can provide ground resolution from 80 meters down to 1 meter.
Satellites for use in imaging typically move around the earth in circular, substantially polar orbits, preferably arranged so that the satellite crosses the equator at approximately the same local sun time on each pass. In this way, data collection takes place at all times and all points under identical light conditions for each season of the year. The orbits are further organised so that complete coverage of the entire surface of the world takes place over a cyclic period typically occupying some 3 to 4 weeks.
In one of the Landsat systems, for example, a line-scanner device is used which continually scans the surface of the earth along a 185-kilometer-wide swath perpendicular to the satellite's orbital motion. By mirror sweep scanning, a number of scan lines are recorded simultaneously in each of a number of spectral bands. Each band is provided with a number of detectors corresponding to variations in the intensity of reflected sunlight. A nominal instantaneous field of view of each detector is typically a square corresponding to a ground resolution cell of 79 meters square. The analogue output of each detector is encoded as digital data. Under certain conditions, objects smaller than the minimum ground resolution can also be recorded. For example, for an object having a reflectance differing significantly from its immediate surroundings, that object is reproduced by the satellite image, even if smaller than the minimum resolution. In the same manner, an object larger than the minimum resolution but having a very low reflectance contrast with its surroundings may not appear on the image. Also reflectance varies from band to band.
Thus in a particular Landsat system, each scan line may contain in excess of 3,000 discrete pixels, and, in a single standard Landsat image, there are typically in excess of 2,000 scan lines. Thus, with the different bands, each Landsat scene is made up of more than 30 million individual observations. Data is collected in digital form, and is thus most suitably processed by computer techniques.
Digital processing techniques also guard against loss of data and ensure faithful rendering of each scene. They also facilitate image restoration, enhancement and classification. Standard corrections and enhancements may be made to suit specific users. Techniques applied in image restoration may include radiometric corrections and geometric corrections. Each pixel may be separately processed for each of the four bands recorded in the original Landsat image, to provide cosmetic improvements in the final appearance of the image. Image enhancement techniques include contrast stretching, density slicing, edge enhancement and spatial-directional filtering. The finally processed data is then transferred onto diapositive film at whatever scale is desired.
Image output is typically in so-called false-colour. The reason for this is that the standard image provided by Landsat and many other satellites has the spectral properties of an infrared colour photograph. The observable differences in colour formation are determined by an object's high reflectance in any of four spectral bands. Thus high reflectance in the green band reproduces as blue, red reflectance reproduces as green, and high infrared reflectance as red. Thus in false-colour, healthy green vegetation generally appears in various tones of red. Green objects will register in blue.
For use by lay readers, false-colour composites are preferably transformed into a normal-colour image. In normal colour, it is far easier to recognise landscape and environmental features. Thus green vegetation is represented by shades of green, and agricultural areas range from green to brown and orange depending on season. Tonal variations in true colour are very carefully controlled to ensure that the different features of the original image can be identified without difficulty. Thus colour scales must be optimised, to enable preparation of suitable colour separations from the original false-colour digital data.
These known techniques are well established and result in satellite image reproduction having varying degrees of technical value and aesthetic appeal. In order to facilitate users in identifying features on reproductions of satellite images, it is also known to print text and other representational material as an overlay on the printed reproduction of the satellite image. The resulting product tends to be somewhat ineffective as a useful means of conveying information, as overlaid material frequently merges with its background, depending on the nature of the features imaged and on the nature and characteristics of the material printed as an overlay.